Introduce getPrefixEndToken to avoid too big context in messages.

pkg/compiler/lib/src/compiler.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 dart2js.compiler_base;
 
 import 'dart:async' show
     EventSink,
     Future;
 
@@ skipping 1777 matching lines @@
     if (begin == null || end == null) {
       // TODO(ahe): We can almost always do better. Often it is only
       // end that is null. Otherwise, we probably know the current
       // URI.
       throw 'Cannot find tokens to produce error message.';
     }
     if (uri == null && currentElement != null) {
       uri = currentElement.compilationUnit.script.resourceUri;
       assert(invariant(currentElement, () {
 
+        bool sameToken(Token token, Token sought) {
+          if (token == sought) return true;
+          if (token.stringValue == '>>' ||
+              token.stringValue == '>>>') {
+            // `>>` and `>>>` are converted to `>` in the parser when needed.
+            return sought.stringValue == '>' &&
+                token.charOffset <= sought.charOffset &&
+                sought.charOffset < token.charEnd;
+          }
+          return false;
+        }
+
         /// Check that [begin] and [end] can be found between [from] and [to].
         validateToken(Token from, Token to) {
           if (from == null || to == null) return true;
           bool foundBegin = false;
           bool foundEnd = false;
           Token token = from;
           while (true) {
-            if (token == begin) {
+            if (sameToken(token, begin)) {
               foundBegin = true;
             }
-            if (token == end) {
+            if (sameToken(token, end)) {
               foundEnd = true;
             }
             if (foundBegin && foundEnd) {
               return true;
             }
             if (token == to || token == token.next || token.next == null) {
               break;
             }
             token = token.next;
           }
@@ skipping 33 matching lines @@
             return validateToken(from, to);
           }
         }
         return true;
       }, message: "Invalid current element: $currentElement [$begin,$end]."));
     }
     return new SourceSpan.fromTokens(uri, begin, end);
   }
 
   SourceSpan spanFromNode(Node node) {
-    return spanFromTokens(node.getBeginToken(), node.getEndToken());
+    return spanFromTokens(node.getBeginToken(), node.getPrefixEndToken());
   }
 
   SourceSpan spanFromElement(Element element) {
     if (element != null && element.sourcePosition != null) {
       return element.sourcePosition;
     }
     while (element != null && element.isSynthesized) {
       element = element.enclosingElement;
     }
     if (element != null &&
@@ skipping 317 matching lines @@
     if (_otherDependencies == null) {
       _otherDependencies = new Setlet<Element>();
     }
     _otherDependencies.add(element.implementation);
   }
 
   Iterable<Element> get otherDependencies {
     return _otherDependencies != null ? _otherDependencies : const <Element>[];
   }
 }