At SSA build-time, change the block information that we attach to loop with aborting bodies.

sdk/lib/_internal/compiler/implementation/elements/elements.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.
 
 library elements;
 
 import 'dart:uri';
 
 // TODO(ahe): Rename prefix to 'api' when VM bug is fixed.
 import '../../compiler.dart' as api_e;
@@ skipping 1915 matching lines @@
   }
 }
 
 class LabelElement extends Element {
   // We store the original label here so it can be returned by [parseNode].
   final Label label;
   final String labelName;
   final TargetElement target;
   bool isBreakTarget = false;
   bool isContinueTarget = false;
-  LabelElement(Label label, this.labelName, this.target,
+  LabelElement(Label label, String labelName, this.target,
                Element enclosingElement)
       : this.label = label,
-        super(label.identifier.source, ElementKind.LABEL, enclosingElement);
+        this.labelName = labelName,
+        // In case of a synthetic label, just use [labelName] for
+        // identifying the element.
+        super(label == null
+                  ? new SourceString(labelName)
+                  : label.identifier.source,
+              ElementKind.LABEL,
+              enclosingElement);
 
   void setBreakTarget() {
     isBreakTarget = true;
     target.isBreakTarget = true;
   }
   void setContinueTarget() {
     isContinueTarget = true;
     target.isContinueTarget = true;
   }
 
@@ skipping 89 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }