Add support for javascript incompatibility warnings.

tests/standalone/javascript_compatibility_errors_test.dart

+// Copyright (c) 2014, 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=--warn_on_javascript_incompatibility --warning_as_error --optimization_counter_threshold=5
+
+import "package:expect/expect.dart";
+
+f(x, y) {
+  // Unoptimized code.
+  1 is double;  /// 00: compile-time error
+  if (1 is double) { x++; }  /// 01: compile-time error
+  try { 1 as double; } on CastError catch (e) { }  /// 02: compile-time error
+  try { var y = 1 as double; } on CastError catch (e) { }  /// 03: compile-time error

[srdjan, 2014/05/08 18:11:05]

Check that "hello" is int does not produce warnings

[regis, 2014/05/09 21:03:42]

I added a whole new section verifying that no warnings are generated.

+  1.0 is int;  /// 04: compile-time error
+  if (1.0 is int) { x++; }  /// 05: compile-time error
+  try { 1.0 as int; } on CastError catch (e) { }  /// 06: compile-time error
+  try { var z = 1.0 as int; } on CastError catch (e) { }  /// 07: compile-time error
+
+  x is double;  /// 10: ok
+  if (x is double) { }  /// 11: ok
+  try { x as double; } on CastError catch (e) { }  /// 12: ok
+  try { var z = x as double; } on CastError catch (e) { }  /// 13: ok
+  y is int;  /// 14: ok
+  if (y is int) { }  /// 15: ok
+  try { y as int; } on CastError catch (e) { }  /// 16: ok
+  try { var z = y as int; } on CastError catch (e) { }  /// 17: ok
+
+  "${1.0}";  /// 20: compile-time error
+  var z = "${1.0}";  /// 21: compile-time error
+  (1.0).toString();  /// 22: ok
+  var z = (1.0).toString();  /// 23: ok
+  "$y";  /// 24: ok
+  var z = "$y";  /// 25: ok
+  y.toString();  /// 26: ok
+  var z = y.toString();  /// 27: ok
+
+  if (x > 10) {
+    // Optimized code.
+    x is double;  /// 30: ok
+    if (x is double) { }  /// 31: ok
+    try { x as double; } on CastError catch (e) { }  /// 32: ok
+    try { var z = x as double; } on CastError catch (e) { }  /// 33: ok
+    y is int;  /// 34: ok
+    if (y is int) { }  /// 35: ok
+    try { y as int; } on CastError catch (e) { }  /// 36: ok
+    try { var z = y as int; } on CastError catch (e) { }  /// 37: ok
+
+    "${1.0}";  /// 40: compile-time error
+    var z = "${1.0}";  /// 41: compile-time error
+    (1.0).toString();  /// 42: ok
+    var z = (1.0).toString();  /// 43: ok
+    "$y";  /// 44: ok
+    var z = "$y";  /// 45: ok
+    y.toString();  /// 46: ok
+    var z = y.toString();  /// 47: ok
+  }
+}
+
+g(x, y) => f(x, y);  // Test inlining calls.
+h(x, y) => g(x, y);
+
+// We don't test for _JavascriptCompatibilityError since it's not visible.
+// It should not be visible since it doesn't exist on dart2js.
+bool isJavascriptCompatibilityError(e) =>
+    e is Error && "$e".contains("javascript incompatibility");
+
+main() {
+  // Since the warning (or error in case of --warning_as_error) is issued at
+  // most once per location, the Expect.throw must guard the whole loop.
+  Expect.throws(
+      () {
+        for (var i = 0; i < 20; i++) {
+          h(i, i * 1.0);
+        }
+      },
+      isJavascriptCompatibilityError);
+}
+