Trying to unbreak dart editor build - new use of rollback messes up code completion

compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java

Index: compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java
diff --git a/compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java b/compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java
index 49f71fbb8949c11bcc43ce33b7fe49855f50ecd0..35901cdefa3d9049ceeed4b4cffa11d374e24da4 100644
--- a/compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java
+++ b/compiler/java/com/google/dart/compiler/parser/CompletionHooksParserBase.java
@@ -434,7 +434,8 @@ public abstract class CompletionHooksParserBase extends AbstractParser {
 
   /**
    * This should only be called when the parser is looking ahead to decide how
-   * to parse something, and this will always be rolled back.
+   * to parse something, and this will always be rolled back without any other {@link #begin()}
+   * statements being called.
    */
   protected void startLookahead() {
     begin();