Add optional message argument to assert statements in the VM.

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 59a61ec4bb9238f98da38db0bd605b63a7c3c224..715720d002b3eac1a436afb1048e0942df4bc1e5 100644
--- a/runtime/lib/errors_patch.dart
+++ b/runtime/lib/errors_patch.dart
@@ -21,14 +21,22 @@ patch class Error {
 
 class _AssertionError extends Error implements AssertionError {
   _AssertionError._create(
-      this._failedAssertion, this._url, this._line, this._column);
+      this._failedAssertion, this._url, this._line, this._column,
+      this.message);
 
-  static _throwNew(int assertionStart, int assertionEnd)
+  static _throwNew(int assertionStart, int assertionEnd, Object message)
       native "AssertionError_throwNew";
 
+  String get _messageString {
+    if (message == null) return "is not true.";
+    if (message is String) return message;
+    return Error.safeToString(message);
+  }
+
   String toString() {
     if (_url == null) {
-      return _failedAssertion;
+      if (message == null) return _failedAssertion;
+      return "'$_failedAssertion' $_messageString";
     }
     var columnInfo = "";
     if (_column > 0) {
@@ -36,19 +44,20 @@ class _AssertionError extends Error implements AssertionError {
       columnInfo = " pos $_column";
     }
     return "'$_url': Failed assertion: line $_line$columnInfo: "
-        "'$_failedAssertion' is not true.";
+           "'$_failedAssertion' $_messageString";

[Lasse Reichstein Nielsen, 2015/08/28 18:51:59]

The format is negotiable.

   }
   final String _failedAssertion;
   final String _url;
   final int _line;
   final int _column;
+  final Object message;
 }
 
 class _TypeError extends _AssertionError implements TypeError {
   _TypeError._create(String url, int line, int column,
                      this._srcType, this._dstType, this._dstName,
-                     this._errorMsg)
-      : super._create("is assignable", url, line, column);
+                     String error_msg)
+      : super._create("is assignable", url, line, column, error_msg);
 
   static _throwNew(int location,
                    Object src_value,
@@ -70,7 +79,7 @@ class _TypeError extends _AssertionError implements TypeError {
 
 
   String toString() {
-    String str = (_errorMsg != null) ? _errorMsg : "";
+    String str = (message != null) ? message : "";
     if ((_dstName != null) && (_dstName.length > 0)) {
       str = "${str}type '$_srcType' is not a subtype of "
             "type '$_dstType' of '$_dstName'.";
@@ -83,7 +92,6 @@ class _TypeError extends _AssertionError implements TypeError {
   final String _srcType;
   final String _dstType;
   final String _dstName;
-  final String _errorMsg;
 }
 
 class _CastError extends Error implements CastError {