Deal with Javascript integer limitations in heap snapshot processing to handle 64-bit target VMs.

runtime/observatory/tests/service/read_stream_test.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+// VMOptions=--compile_all --error_on_bad_type --error_on_bad_override
+
+import 'package:observatory/object_graph.dart';
+import 'package:expect/expect.dart';
+import 'dart:typed_data';
+
+testRoundTrip(final int n) {
+  var bytes = [];
+  var remaining = n;
+  while (remaining > 127) {
+    bytes.add(remaining & 127);
+    remaining = remaining >> 7;
+  }
+  bytes.add(remaining + 128);
+
+  print("Encoded $n as $bytes");
+
+  var typedBytes = new ByteData.view(new Uint8List.fromList(bytes).buffer);
+  var stream = new ReadStream([typedBytes]);
+  stream.readUnsigned();
+
+  Expect.equals(n == 0, stream.isZero);
+
+  Expect.equals((n >>  0) & 0xFFFFFFF, stream.low);
+  Expect.equals((n >> 28) & 0xFFFFFFF, stream.mid);
+  Expect.equals((n >> 56) & 0xFFFFFFF, stream.high);
+
+  const kMaxUint32 = (1 << 32) - 1;
+  if (n > kMaxUint32) {
+    Expect.equals(kMaxUint32, stream.clampedUint32);
+  } else {
+    Expect.equals(n, stream.clampedUint32);
+  }
+
+  Expect.equals(bytes.length, stream.position);
+}
+
+main() {
+  const kMaxUint64 = (1 << 64) - 1;
+
+  var n = 3;
+  while (n < kMaxUint64) {
+    testRoundTrip(n);
+    n <<= 1;
+  }
+
+  n = 5;
+  while (n < kMaxUint64) {
+    testRoundTrip(n);
+    n <<= 1;
+  }
+}