Add token invariant to DiagnosticReporter

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 1774 matching lines @@
 
   SourceSpan spanFromTokens(Token begin, Token end, [Uri uri]) {
     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, () {
+
+        /// 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) {
+              foundBegin = true;
+            }
+            if (token == end) {
+              foundEnd = true;
+            }
+            if (foundBegin && foundEnd) {
+              return true;
+            }
+            if (token == to || token == token.next || token.next == null) {
+              break;
+            }
+            token = token.next;
+          }
+
+          // Create a good message for when the tokens were not found.
+          StringBuffer sb = new StringBuffer();
+          sb.write('Invalid current element: $currentElement. ');
+          sb.write('Looking for ');
+          sb.write('[${begin} (${begin.hashCode}),');
+          sb.write('${end} (${end.hashCode})] in');
+
+          token = from;
+          while (true) {
+            sb.write('\n ${token} (${token.hashCode})');
+            if (token == to || token == token.next || token.next == null) {
+              break;
+            }
+            token = token.next;
+          }
+          return sb.toString();
+        }
+
+        if (currentElement.enclosingClass != null &&
+            currentElement.enclosingClass.isEnumClass) {
+          // Enums ASTs are synthesized (and give messed up messages).
+          return true;
+        }
+
+        if (currentElement is AstElement) {
+          AstElement astElement = currentElement;
+          if (astElement.hasNode) {
+            Token from = astElement.node.getBeginToken();
+            Token to = astElement.node.getEndToken();
+            if (astElement.metadata.isNotEmpty) {
+              from = astElement.metadata.first.beginToken;
+            }
+            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());
   }
 
   SourceSpan spanFromElement(Element element) {
     if (element != null && element.sourcePosition != null) {
@@ skipping 323 matching lines @@
     if (_otherDependencies == null) {
       _otherDependencies = new Setlet<Element>();
     }
     _otherDependencies.add(element.implementation);
   }
 
   Iterable<Element> get otherDependencies {
     return _otherDependencies != null ? _otherDependencies : const <Element>[];
   }
 }