Add optional message to assert in Dart2js.

sdk/lib/_internal/js_runtime/lib/js_helper.dart

Index: sdk/lib/_internal/js_runtime/lib/js_helper.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/js_helper.dart b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
index 91049cda0fe66872d542cdc2d79fdb2ac2b06f45..658123e4cd86a49613b02d1d935f8db85137980e 100644
--- a/sdk/lib/_internal/js_runtime/lib/js_helper.dart
+++ b/sdk/lib/_internal/js_runtime/lib/js_helper.dart
@@ -3298,8 +3298,11 @@ class FallThroughErrorImplementation extends FallThroughError {
 
 /**
  * Helper function for implementing asserts. The compiler treats this specially.
+ *
+ * Returns the negation of the condition. That is: `true` if the assert should
+ * fail.
  */
-void assertHelper(condition) {
+bool assertTest(condition) {
   // Do a bool check first because it is common and faster than 'is Function'.
   if (condition is !bool) {
     if (condition is Function) condition = condition();
@@ -3307,9 +3310,23 @@ void assertHelper(condition) {
       throw new TypeErrorImplementation(condition, 'bool');
     }
   }
-  // Compare to true to avoid boolean conversion check in checked
-  // mode.
-  if (true != condition) throw new AssertionError();
+  return !condition;
+}
+
+/**
+ * Helper function for implementing asserts with messages.
+ * The compiler treats this specially.
+ */
+void assertThrow(Object message) {
+  throw new _AssertionError(message);
+}
+
+/**
+ * Helper function for implementing asserts without messages.
+ * The compiler treats this specially.
+ */
+void assertHelper(condition) {
+  if (assertTest(condition)) throw new AssertError();

[sra1, 2015/09/15 23:52:55]

AssertionError()

 }
 
 /**
@@ -3972,3 +3989,10 @@ void badMain() {
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }
+
+class _AssertionError extends AssertionError {
+  final _message;
+  _AssertionError(this._message);
+
+  String toString() => "Assertion failed: " + Error.safeToString(_message);
+}