Don't allow elements of a DartBinaryExpression to be null.

compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java

Index: compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
diff --git a/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java b/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
index 592ca63d0515031b4033666e57ecd619a513ac87..96d1c0d45a5f99e71c978ca72d613f0384f14535 100644
--- a/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
+++ b/compiler/java/com/google/dart/compiler/ast/DartBinaryExpression.java
@@ -22,8 +22,8 @@ public class DartBinaryExpression extends DartExpression implements ElementRefer
     assert op.isBinaryOperator() : op;
 
     this.op = op;
-    this.arg1 = becomeParentOf(arg1);
-    this.arg2 = becomeParentOf(arg2);
+    this.arg1 = becomeParentOf(arg1 != null ? arg1 : new DartSyntheticErrorExpression());

[mmendez, 2011/12/19 20:01:37]

Nit: should you be passing the token string to the DartSyntheticErrorExpression
constructor.

[codefu, 2011/12/19 23:24:18]

There is no tokenString in this case as the argument is null.  Two of the other
3 pass either an empty string or a null; all three deal with string
interpolation.  I can update it to pass an empty string.

+    this.arg2 = becomeParentOf(arg2 != null ? arg2 : new DartSyntheticErrorExpression());
   }
 
   public DartExpression getArg1() {