Implement new constant expressions

editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java

Index: editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java
diff --git a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java
index c7a0fd3863dac589a57f1bd8a5c0362652597bc3..37f6fc06daa8e14ad68e555b6d723518284252e4 100644
--- a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java
+++ b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/constant/ValidResult.java
@@ -273,6 +273,15 @@ public class ValidResult extends EvaluationResultImpl {
   }
 
   @Override
+  public EvaluationResultImpl stringLength(TypeProvider typeProvider, Expression node) {
+    try {
+      return valueOf(value.stringLength(typeProvider));
+    } catch (EvaluationException exception) {
+      return error(node, exception.getErrorCode());
+    }
+  }
+
+  @Override
   public EvaluationResultImpl times(TypeProvider typeProvider, BinaryExpression node,
       EvaluationResultImpl rightOperand) {
     return rightOperand.timesValid(typeProvider, node, this);