Nice messages on uncaught exceptions.

dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart

Index: dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
diff --git a/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart b/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
index cc6d052cce473a4d293d4ddbcea5d028b473526d..1e32578247bd6305e7439521a0a3eb4d68986c5a 100644
--- a/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
+++ b/dart/sdk/lib/_internal/compiler/implementation/lib/js_helper.dart
@@ -951,30 +951,81 @@ class MathNatives {
 }
 
 /**
- * Throws the given Dart object as an exception by wrapping it in a
- * proper JavaScript error object and then throwing that. That gives
- * us a reasonable stack trace on most JavaScript implementations. The
- * code in [unwrapException] deals with getting the original Dart
+ * Wrap the given Dart object and record a stack trace.
+ *
+ * The code in [unwrapException] deals with getting the original Dart
  * object out of the wrapper again.
  */
 $throw(ex) {
   if (ex == null) ex = const NullThrownError();
-  var jsError = JS('var', r'new Error()');
-  JS('void', r'#.name = #', jsError, ex);
-  JS('void', r'#.description = #', jsError, ex);
-  JS('void', r'#.dartException = #', jsError, ex);
-  JS('void', r'#.toString = #', jsError,
-     DART_CLOSURE_TO_JS(toStringWrapper));
-  JS('void', r'throw #', jsError);
+  var wrapper = new DartError(ex);
+
+  if (JS('bool', 'Error.captureStackTrace')) {

[sra1, 2013/01/15 11:48:43]

!!Error.captureStackTrace

Otherwise it depends too much on how the dart2js inserts bool checks in checked
mode.  It is possible that in checked mode that the JS function or 'undefined'
value will be flagged as not bool.

[ahe, 2013/01/15 12:56:01]

Done.

+    // Use V8 API for recording a "fast" stack trace (this installs a
+    // "stack" property getter on [wrapper]).
+    JS('void', r'Error.captureStackTrace(#, #)',
+       wrapper, RAW_DART_FUNCTION_REF($throw));
+  } else {
+    // Slower way to create a stack trace which works on some
+    // browsers, but may simply evaluate to null.
+    String stackTrace = JS('', 'new Error().stack');
+    // Otherwise, produce a stack trace and record it in the wrapper.

[ngeoffray, 2013/01/15 09:15:42]

I think you can merge this comment with the new one above.

[ahe, 2013/01/15 10:34:30]

Done.

+    JS('void', '#.stack = #', wrapper, stackTrace);
+  }
+  return wrapper;
 }
 
 /**
- * This method is installed as JavaScript toString method on exception
- * objects in [$throw]. So JavaScript 'this' binds to an instance of
- * JavaScript Error to which we have added a property 'dartException'
- * which holds a Dart object.
+ * Wrapper class for throwing exceptions.
  */
-toStringWrapper() => JS('', r'this.dartException').toString();
+class DartError {
+  /// The Dart object (or primitive JavaScript value) which was thrown.
+  final dartException;
+
+  DartError(this.dartException) {
+    // Install a toString method that the JavaScript system will call
+    // to format uncaught exceptions.
+    JS('void', '#.toString = #', this, DART_CLOSURE_TO_JS(toStringWrapper));
+  }
+
+  /**
+   * V8/Chrome installs a property getter, "stack", when calling
+   * Error.captureStackTrace (see [$throw]). In [$throw], we make sure
+   * that this property is always set.
+   */
+  String get stack => JS('', '#.stack', this);
+
+  /**
+   * This method can be invoked by calling toString from
+   * JavaScript. See the constructor of this class.
+   *
+   * We only expect this method to be called (indirectly) by the
+   * browser when an uncaught exception occurs. Instance of this class
+   * should never escape into Dart code (except for [$throw] above).
+   */
+  String toString() {
+    // If Error.captureStackTrace is available, accessing stack from
+    // this method would cause recursion because the stack property
+    // (on this object) is actually a getter which calls toString on
+    // this object (via the wrapper installed in this class'
+    // constructor). Fortunately, both Chrome and d8 prints the stack
+    // trace and Chrome even applies source maps to the stack
+    // trace. Remeber, this method is only ever invoked by the browser
+    // when an uncaught exception occurs.
+    if (JS('bool', 'Error.captureStackTrace') || (stack == null)) {

[sra1, 2013/01/15 11:48:43]

!!

[ahe, 2013/01/15 12:56:01]

Done.

+      return dartException.toString();
+    } else {
+      return '$dartException\n$stack';
+    }
+  }
+
+  /**
+   * This method is installed as JavaScript toString method on
+   * [DartError].  So JavaScript 'this' binds to an instance of
+   * DartError.
+   */
+  static toStringWrapper() => JS('', r'this').toString();

[sra1, 2013/01/15 11:48:43]

Make this a private name?

[ahe, 2013/01/15 12:56:01]

Why?

+}
 
 makeLiteralListConst(list) {
   JS('bool', r'#.immutable$list = #', list, true);