Issue 4900. Fix for for-in cycle and field as identifier

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

Index: compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
diff --git a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
index beccb3422bf5e0d7b8d8a34df66e55a1ae3edd13..8efe4b6af7478b637c8c18c6435859a27ba2bc05 100644
--- a/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
+++ b/compiler/java/com/google/dart/compiler/type/TypeAnalyzer.java
@@ -1602,13 +1602,17 @@ public class TypeAnalyzer implements DartCompilationPhase {
     @Override
     public Type visitForInStatement(DartForInStatement node) {
       Type variableType;
-      VariableElement variableElement;
+      VariableElement variableElement = null;
       if (node.introducesVariable()) {
         variableType = typeOf(node.getVariableStatement());
         variableElement = node.getVariableStatement().getVariables().get(0).getElement();
       } else {
         variableType = typeOf(node.getIdentifier());
-        variableElement = (VariableElement) node.getIdentifier().getElement();
+        // in most cases variable, but sometimes field
+        NodeElement identifierElement = node.getIdentifier().getElement();
+        if (identifierElement instanceof VariableElement) {
+          variableElement = (VariableElement) identifierElement;
+        }
       }
       // prepare Iterable type
       DartExpression iterableExpression = node.getIterable();
@@ -1639,7 +1643,7 @@ public class TypeAnalyzer implements DartCompilationPhase {
       // visit body with inferred variable type
       VariableElementsRestorer variableRestorer = new VariableElementsRestorer();
       try {
-        if (elementType != null) {
+        if (variableElement != null && elementType != null) {
           variableRestorer.setType(variableElement, elementType);
         }
         BlockTypeContext blockTypeContext = new BlockTypeContext();