Nice messages on uncaught exceptions.

dart/sdk/lib/_internal/compiler/implementation/js_backend/emitter.dart

 // Copyright (c) 2012, 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.
 
 part of js_backend;
 
 /**
  * A function element that represents a closure call. The signature is copied
  * from the given element.
  */
@@ skipping 372 matching lines @@
   var sentinelUndefined = {};
   var sentinelInProgress = {};
   prototype[fieldName] = sentinelUndefined;
   prototype[getterName] = function() {
     var result = $isolate[fieldName];
     try {
       if (result === sentinelUndefined) {
         $isolate[fieldName] = sentinelInProgress;
         try {
           result = $isolate[fieldName] = lazyValue();
-        } catch (e) {
-          if ($isolate[fieldName] === sentinelInProgress) {
-            $isolate[fieldName] = null;
+        } finally {
+""" // Use try-finally, not try-catch/throw as it destroys the stack trace.
+"""
+          if (result === sentinelUndefined) {
+            if ($isolate[fieldName] === sentinelInProgress) {
+              $isolate[fieldName] = null;
+            }
           }
-          throw e;
         }
       } else if (result === sentinelInProgress) {
         $cyclicThrow(staticName);
       }
       return result;
     } finally {
       $isolate[getterName] = getter;
     }
   };""";
   }
@@ skipping 1759 matching lines @@
 """;
 const String HOOKS_API_USAGE = """
 // The code supports the following hooks:
 // dartPrint(message)   - if this function is defined it is called
 //                        instead of the Dart [print] method.
 // dartMainRunner(main) - if this function is defined, the Dart [main]
 //                        method will not be invoked directly.
 //                        Instead, a closure that will invoke [main] is
 //                        passed to [dartMainRunner].
 """;