Make a single labeled statement hold all its labels.

lib/compiler/implementation/resolver.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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   Type getType(TypeAnnotation annotation);
 }
 
@@ skipping 457 matching lines @@
   }
 }
 
 interface LabelScope {
   LabelScope get outer();
   LabelElement lookup(String label);
 }
 
 class LabeledStatementLabelScope implements LabelScope {
   final LabelScope outer;
-  final LabelElement label;
-  LabeledStatementLabelScope(this.outer, this.label);
+  final Map<String, LabelElement> labels;
+  LabeledStatementLabelScope(this.outer, this.labels);

[ahe, 2012/05/14 10:11:40]

My guess is that this scope is slower than what you have now. However, it is
saved by labels not being common, so it probably doesn't matter.

[Lasse Reichstein Nielsen, 2012/05/14 10:44:59]

True. It's probably slower if there is only one label, which is the "common"
case.
If it turns out to be a measurable problem, we can introduce a simpler scope for
that case.

   LabelElement lookup(String labelName) {
-    if (this.label.labelName == labelName) return label;
+    LabelElement label = labels[labelName];
+    if (label !== null) return label;
     return outer.lookup(labelName);
   }
 }
 
 class SwitchLabelScope implements LabelScope {
   final LabelScope outer;
   final Map<String, LabelElement> caseLabels;
 
   SwitchLabelScope(this.outer, this.caseLabels);
 
@@ skipping 27 matching lines @@
 
   LabelElement lookupLabel(String label) {
     return labels.lookup(label);
   }
   TargetElement currentBreakTarget() =>
     breakTargetStack.isEmpty() ? null : breakTargetStack.head;
 
   TargetElement currentContinueTarget() =>
     continueTargetStack.isEmpty() ? null : continueTargetStack.head;
 
-  void enterLabelScope(LabelElement element) {
-    labels = new LabeledStatementLabelScope(labels, element);
+  void enterLabelScope(Map<String, LabelElement> elements) {
+    labels = new LabeledStatementLabelScope(labels, elements);
     nestingLevel++;
   }
 
   void exitLabelScope() {
     nestingLevel--;
     labels = labels.outer;
   }
 
   void enterLoop(TargetElement element) {
     breakTargetStack = breakTargetStack.prepend(element);
@@ skipping 697 matching lines @@
       // The variable declaration is either not an identifier, not a
       // declaration, or it's declaring more than one variable.
       error(node.declaredIdentifier, MessageKind.INVALID_FOR_IN, []);
     }
   }
 
   visitLabel(Label node) {
   }
 
   visitLabeledStatement(LabeledStatement node) {
-    String labelName = node.label.slowToString();
-    LabelElement existingElement = statementScope.lookupLabel(labelName);
-    if (existingElement !== null) {
-      warning(node.label, MessageKind.DUPLICATE_LABEL, [labelName]);
-      warning(existingElement.label, MessageKind.EXISTING_LABEL, [labelName]);
+    Statement body = node.statement;
+    TargetElement targetElement = getOrCreateTargetElement(body);
+    Map<String, LabelElement> labelElements = <LabelElement>{};
+    for (Label label in node.labels) {
+      String labelName = label.slowToString();
+      if (labelElements.containsKey(labelName)) continue;
+      LabelElement element = targetElement.addLabel(label, labelName);
+      labelElements[labelName] = element;
     }
-    Node body = node.getBody();
-    TargetElement targetElement = getOrCreateTargetElement(body);
-
-    LabelElement element = targetElement.addLabel(node.label, labelName);
-    statementScope.enterLabelScope(element);
+    statementScope.enterLabelScope(labelElements);
     visit(node.statement);
     statementScope.exitLabelScope();
-    if (element.isTarget) {
-      mapping[node.label] = element;
-    } else {
-      warning(node.label, MessageKind.UNUSED_LABEL, [labelName]);
-    }
+    labelElements.forEach((String labelName, LabelElement element) {
+      if (element.isTarget) {
+        mapping[element.label] = element;
+      } else {
+        warning(element.label, MessageKind.UNUSED_LABEL, [labelName]);
+      }
+    });
     if (!targetElement.isTarget && mapping[body] === targetElement) {
       // If the body is itself a break or continue for another target, it
       // might have updated its mapping to the target it actually does target.
       mapping.remove(body);
     }
   }
 
   visitLiteralMap(LiteralMap node) {
     node.visitChildren(this);
   }
@@ skipping 663 matching lines @@
 
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) {
     return library.find(name);
   }
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
 }