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diff --git a/devtools/shared/heapsnapshot/tests/xpcshell/Census.sys.mjs b/devtools/shared/heapsnapshot/tests/xpcshell/Census.sys.mjs
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+++ b/devtools/shared/heapsnapshot/tests/xpcshell/Census.sys.mjs
@@ -0,0 +1,176 @@
+// Functions for checking results returned by
+// Debugger.Memory.prototype.takeCensus and
+// HeapSnapshot.prototype.takeCensus. Adapted from js/src/jit-test/lib/census.js.
+
+export const Census = {};
+function dumpn(msg) {
+  dump("DBG-TEST: Census.jsm: " + msg + "\n");
+}
+
+// Census.walkCensus(subject, name, walker)
+//
+// 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(): Called after we have called 'enter' on every property of the
+//   subject.
+//
+// - 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.
+Census.walkCensus = (subject, name, walker) => walk(subject, name, walker, 0);
+function walk(subject, name, walker, count) {
+  if (typeof subject === "object") {
+    dumpn(name);
+    for (const prop in subject) {
+      count = walk(
+        subject[prop],
+        name + "[" + uneval(prop) + "]",
+        walker.enter(prop),
+        count
+      );
+    }
+    walker.done();
+  } else {
+    dumpn(name + " = " + uneval(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 => {
+    if (elt !== 0) {
+      throw new Error("Census mismatch: expected zero, found " + elt);
+    }
+  },
+};
+
+function expectedObject() {
+  throw new Error(
+    "Census mismatch: subject has leaf where basis has nested object"
+  );
+}
+
+function expectedLeaf() {
+  throw new Error(
+    "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") {
+      const unvisited = new Set(Object.getOwnPropertyNames(basis));
+      return {
+        enter: prop => {
+          unvisited.delete(prop);
+          if (prop in basis) {
+            return makeWalker(basis[prop]);
+          }
+
+          return extra(prop);
+        },
+
+        done: () => unvisited.forEach(prop => missing(prop, basis[prop])),
+        check: expectedObject,
+      };
+    }
+
+    return {
+      enter: expectedLeaf,
+      done: expectedLeaf,
+      check: elt => compare(elt, basis),
+    };
+  };
+}
+
+function missingProp(prop) {
+  throw new Error(
+    "Census mismatch: subject lacks property present in basis: " + prop
+  );
+}
+
+function extraProp(prop) {
+  throw new Error(
+    "Census mismatch: subject has property not present in basis: " + prop
+  );
+}
+
+// Return a walker that checks that the subject census has counts all equal to
+// |basis|.
+Census.assertAllEqual = makeBasisChecker({
+  compare: (a, b) => {
+    if (a !== b) {
+      throw new Error("Census mismatch: expected " + a + " got " + b);
+    }
+  },
+  missing: missingProp,
+  extra: extraProp,
+});
+
+function ok(val) {
+  if (!val) {
+    throw new Error("Census mismatch: expected truthy, got " + val);
+  }
+}
+
+// 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) => ok(subject >= basis),
+  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) => ok(subject <= basis),
+  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, base) => ok(Math.abs(subject - base) <= fudge),
+    missing: missingProp,
+    extra: () => Census.walkAnything,
+  })(basis);
+};