Working implementation of #29153 range check in as-casts.

runtime/lib/errors_patch.dart

Index: runtime/lib/errors_patch.dart
diff --git a/runtime/lib/errors_patch.dart b/runtime/lib/errors_patch.dart
index f26d4d4e238feb6c54a9618a043040da2ff5bd8e..1f57ad3d1fee038a37be4d0e6fdd760e806ff1bc 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -395,3 +395,21 @@ class _CompileTimeError extends Error {
   _CompileTimeError(this._errorMsg);
   String toString() => _errorMsg;
 }
+
+dynamic _classRangeAssert(int position, String InTypeCast, dynamic instance,
+    _Type type, int cid, int lowerLimit, int upperLimit) {
+  if ((cid < lowerLimit || cid > upperLimit) && instance != null) {
+    _TypeError._throwNew(0, instance, type, InTypeCast, "");

[rmacnak, 2017/04/10 21:07:23]

0 -> position.

It's safe to say "in type type" here. The string literal will be canonicalized
to the same object as the predefined symbol.

+  }
+
+  return instance;
+}
+
+dynamic _classIdEqualsAssert(int position, String InTypeCast, dynamic instance,
+    _Type type, int cid, int otherCid) {
+  if (cid != otherCid && instance != null) {
+    _TypeError._throwNew(0, instance, type, InTypeCast, "");
+  }
+
+  return instance;
+}