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diff --git a/js/src/jit-test/lib/census.js b/js/src/jit-test/lib/census.js
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+// Functions for checking results returned by Debugger.Memory.prototype.takeCensus.
+
+const Census = {};
+
+(function () {
+
+  // Census.walkCensus(subject, name, walker[, ignore])
+  //
+  // Use |walker| to check |subject|, a census object of the sort returned by
+  // Debugger.Memory.prototype.takeCensus: a tree of objects with integers at the
+  // leaves. Use |name| as the name for |subject| in diagnostic messages. Return
+  // the number of leaves of |subject| we visited.
+  //
+  // A walker is an object with three methods:
+  //
+  // - enter(prop): Return the walker we should use to check the property of the
+  //   subject census named |prop|. This is for recursing into the subobjects of
+  //   the subject.
+  //
+  // - done(ignore): Called after we have called 'enter' on every property of
+  //   the subject. Passed the |ignore| set of properties.
+  //
+  // - check(value): Check |value|, a leaf in the subject.
+  //
+  // Walker methods are expected to simply throw if a node we visit doesn't look
+  // right.
+  //
+  // The optional |ignore| parameter allows you to specify a |Set| of property
+  // names which should be ignored. The walker will not traverse such
+  // properties.
+  Census.walkCensus = (subject, name, walker, ignore = new Set()) =>
+    walk(subject, name, walker, ignore, 0);
+
+  function walk(subject, name, walker, ignore, count) {
+    if (typeof subject === 'object') {
+      print(name);
+      for (let prop in subject) {
+        if (ignore.has(prop)) {
+          continue;
+        }
+        count = walk(subject[prop],
+                     name + "[" + JSON.stringify(prop) + "]",
+                     walker.enter(prop),
+                     ignore,
+                     count);
+      }
+      walker.done(ignore);
+    } else {
+      print(name + " = " + JSON.stringify(subject));
+      walker.check(subject);
+      count++;
+    }
+
+    return count;
+  }
+
+  // A walker that doesn't check anything.
+  Census.walkAnything = {
+    enter: () => Census.walkAnything,
+    done: () => undefined,
+    check: () => undefined
+  };
+
+  // A walker that requires all leaves to be zeros.
+  Census.assertAllZeros = {
+    enter: () => Census.assertAllZeros,
+    done: () => undefined,
+    check: elt => assertEq(elt, 0)
+  };
+
+  function expectedObject() {
+    throw "Census mismatch: subject has leaf where basis has nested object";
+  }
+
+  function expectedLeaf() {
+    throw "Census mismatch: subject has nested object where basis has leaf";
+  }
+
+  // Return a function that, given a 'basis' census, returns a census walker that
+  // compares the subject census against the basis. The returned walker calls the
+  // given |compare|, |missing|, and |extra| functions as follows:
+  //
+  // - compare(subjectLeaf, basisLeaf): Check a leaf of the subject against the
+  //   corresponding leaf of the basis.
+  //
+  // - missing(prop, value): Called when the subject is missing a property named
+  //   |prop| which is present in the basis with value |value|.
+  //
+  // - extra(prop): Called when the subject has a property named |prop|, but the
+  //   basis has no such property. This should return a walker that can check
+  //   the subject's value.
+  function makeBasisChecker({compare, missing, extra}) {
+    return function makeWalker(basis) {
+      if (typeof basis === 'object') {
+        var unvisited = new Set(Object.getOwnPropertyNames(basis));
+        return {
+          enter: prop => {
+            unvisited.delete(prop);
+            if (prop in basis) {
+              return makeWalker(basis[prop]);
+            } else {
+              return extra(prop);
+            }
+          },
+
+          done: ignore => [...unvisited].filter(p => !ignore.has(p)).forEach(p => missing(p, basis[p])),
+          check: expectedObject
+        };
+      } else {
+        return {
+          enter: expectedLeaf,
+          done: expectedLeaf,
+          check: elt => compare(elt, basis)
+        };
+      }
+    };
+  }
+
+  function missingProp(prop) {
+    throw "Census mismatch: subject lacks property present in basis: " + JSON.stringify(prop);
+  }
+
+  function extraProp(prop) {
+    throw "Census mismatch: subject has property not present in basis: " + JSON.stringify(prop);
+  }
+
+  // Return a walker that checks that the subject census has counts all equal to
+  // |basis|.
+  Census.assertAllEqual = makeBasisChecker({
+    compare: assertEq,
+    missing: missingProp,
+    extra: extraProp
+  });
+
+  // Return a walker that checks that the subject census has at least as many
+  // items of each category as |basis|.
+  Census.assertAllNotLessThan = makeBasisChecker({
+    compare: (subject, basis) => assertEq(subject >= basis, true),
+    missing: missingProp,
+    extra: () => Census.walkAnything
+  });
+
+  // Return a walker that checks that the subject census has at most as many
+  // items of each category as |basis|.
+  Census.assertAllNotMoreThan = makeBasisChecker({
+    compare: (subject, basis) => assertEq(subject <= basis, true),
+    missing: missingProp,
+    extra: () => Census.walkAnything
+  });
+
+  // Return a walker that checks that the subject census has within |fudge|
+  // items of each category of the count in |basis|.
+  Census.assertAllWithin = function (fudge, basis) {
+    return makeBasisChecker({
+      compare: (subject, basis) => assertEq(Math.abs(subject - basis) <= fudge, true),
+      missing: missingProp,
+      extra: () => Census.walkAnything
+    })(basis);
+  }
+
+})();