Stop looking for a start token if the last token is detected.

editor/tools/plugins/com.google.dart.tools.ui/src/com/google/dart/tools/ui/text/folding/DartFoldingStructureProvider.java

 /*
  * Copyright (c) 2013, the Dart project authors.
  * 
  * Licensed under the Eclipse Public License v1.0 (the "License"); you may not use this file except
  * in compliance with the License. You may obtain a copy of the License at
  * 
  * http://www.eclipse.org/legal/epl-v10.html
  * 
  * Unless required by applicable law or agreed to in writing, software distributed under the License
  * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
@@ skipping 226 matching lines @@
     }
 
     private ASTNode getFirstRef() {
       return firstRef;
     }
 
     private ProjectionAnnotationModel getModel() {
       return model;
     }
 
-    private TokenStream getScanner() {
-      return getScanner(0);
-    }
-
-    private TokenStream getScanner(int start) {
+    private TokenStream getScanner(int start) throws InvalidSourceException {
       tokenStream.begin(start);
       return tokenStream;
     }
 
     private boolean hasHeaderComment() {
       return hasHeaderComment;
     }
 
     private void setFirstRef(ASTNode type) {
       if (hasFirstRef()) {
@@ skipping 296 matching lines @@
       scanner = new StringScanner(null, source, listener);
       if (errorFound[0]) {
         throw new InvalidSourceException();
       } else {
         firstToken = scanner.tokenize();
         currentToken = firstToken;
         begin = 0;
       }
     }
 
-    void begin(int start) {
+    void begin(int start) throws InvalidSourceException {
       if (start == begin) {
         return;
       }
       if (start < begin) {
         begin = 0;
         currentToken = firstToken;
       }
+      Token prev = currentToken;
       while (begin < start) {
         currentToken = currentToken.getNext();
+        if (currentToken == prev) {
+          throw new InvalidSourceException();
+        }
+        prev = currentToken;
         begin = currentToken.getOffset();
       }
     }
 
     Token next() {
       Token next = currentToken;
       currentToken = currentToken.getNext();
       return next;
     }
   }
@@ skipping 224 matching lines @@
     List<IRegion> regions = new ArrayList<IRegion>();
     if (!ctx.hasFirstRef()) {
       ctx.setFirstRef(reference);
       IRegion headerComment = computeHeaderComment(ctx);
       if (headerComment != null) {
         regions.add(headerComment);
         ctx.setHasHeaderComment();
       }
     }
     int shift = range.getOffset();
-    TokenStream scanner = ctx.getScanner(shift);
+    TokenStream scanner;
+    try {
+      scanner = ctx.getScanner(shift);
+    } catch (InvalidSourceException ex) {
+      return new IRegion[0];
+    }
     int start = shift;
     Token token = scanner.next();
     start = token.getOffset();
     Token comment = token.getPrecedingComments();
     IDocument doc = dartEditor.getDocumentProvider().getDocument(dartEditor.getEditorInput());
     while (comment != null) {
       int s = comment.getOffset();
       int l = comment.getLength();
       if (comment.getLexeme().startsWith("//")) {
         Token nextComment = comment.getNext();
@@ skipping 134 matching lines @@
         }
       };
       v.visitAllNodes(parent);
     } catch (InvalidSourceException x) {
       DartToolsPlugin.log(x);
     }
   }
 
   private IRegion computeHeaderComment(FoldingStructureComputationContext ctx) {
     // search at most up to the first element
-    TokenStream scanner = ctx.getScanner();
+    TokenStream scanner;
+    try {
+      scanner = ctx.getScanner(0);
+    } catch (InvalidSourceException ex) {
+      return null;
+    }
     int headerStart = -1;
     int headerEnd = -1;
     boolean foundComment = false;
     Token terminal = scanner.next();
     Token comment = terminal.getPrecedingComments();
     while (comment != null) {
       if (!foundComment) {
         headerStart = comment.getOffset();
       }
       headerEnd = comment.getEnd();
@@ skipping 281 matching lines @@
     match(deletions, additions, updates, ctx);
     Annotation[] deletedArray = deletions.toArray(new Annotation[deletions.size()]);
     Annotation[] changedArray = updates.toArray(new Annotation[updates.size()]);
     ctx.getModel().modifyAnnotations(deletedArray, additions, changedArray);
     try {
       ctx.setScannerSource(""); // clear token stream
     } catch (InvalidSourceException e1) {
     }
   }
 }