Support argument definition test operator (issue 4264)

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

Index: compiler/java/com/google/dart/compiler/ast/DartUnaryExpression.java
===================================================================
--- compiler/java/com/google/dart/compiler/ast/DartUnaryExpression.java	(revision 11387)
+++ compiler/java/com/google/dart/compiler/ast/DartUnaryExpression.java	(working copy)
@@ -20,7 +20,7 @@
   private MethodNodeElement element;
 
   public DartUnaryExpression(Token operator, int operatorOffset, DartExpression arg, boolean isPrefix) {
-    assert operator.isUnaryOperator() || operator == Token.SUB;
+    assert operator.isUnaryOperator() || operator == Token.SUB || operator == Token.CONDITIONAL;
     this.operator = operator;
     this.operatorOffset = operatorOffset;
     this.arg = becomeParentOf(arg);