Protect against concurrent modification exceptions in a new version of guava.

compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java

Index: compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
===================================================================
--- compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java	(revision 16339)
+++ compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java	(working copy)
@@ -125,6 +125,7 @@
 import com.google.dart.compiler.type.InterfaceType.Member;
 import com.google.dart.compiler.util.apache.ObjectUtils;
 
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -3229,7 +3230,9 @@
         }
 
         // All remaining methods are unimplemented.
-        for (String name : superMembers.keys()) {
+        List<String> keys = new ArrayList<String>(superMembers.keys());
+        for (int i = 0; i < keys.size(); i++) {
+          String name = keys.get(i);
           Collection<Element> elements = superMembers.removeAll(name);
           for (Element element : elements) {
             if (!element.getModifiers().isStatic()) {