Implement new constant expressions

editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java

Index: editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java
diff --git a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java
index 1b5f1bf74e8771ed6ce66709c4326cb104d7f8d7..6e21041afd76d2e4f40138bb9607a83fcffdad5b 100644
--- a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java
+++ b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/InstanceState.java
@@ -29,7 +29,10 @@ public abstract class InstanceState {
    * @return the result of invoking the '+' operator on this object with the given argument
    * @throws EvaluationException if the operator is not appropriate for an object of this kind
    */
-  public NumState add(InstanceState rightOperand) throws EvaluationException {
+  public InstanceState add(InstanceState rightOperand) throws EvaluationException {
+    if (this instanceof StringState || rightOperand instanceof StringState) {
+      return concatenate(rightOperand);
+    }
     assertNumOrNull(this);
     assertNumOrNull(rightOperand);
     throw new EvaluationException(CompileTimeErrorCode.INVALID_CONSTANT);
@@ -93,6 +96,7 @@ public abstract class InstanceState {
    * @throws EvaluationException if the operator is not appropriate for an object of this kind
    */
   public StringState concatenate(InstanceState rightOperand) throws EvaluationException {
+    assertString(rightOperand);
     throw new EvaluationException(CompileTimeErrorCode.INVALID_CONSTANT);
   }
 
@@ -364,6 +368,17 @@ public abstract class InstanceState {
   }
 
   /**
+   * Return the result of invoking the 'length' getter on this object.
+   * 
+   * @return the result of invoking the 'length' getter on this object
+   * @throws EvaluationException if the operator is not appropriate for an object of this kind
+   */
+  public IntState stringLength() throws EvaluationException {
+    assertString(this);
+    throw new EvaluationException(CompileTimeErrorCode.INVALID_CONSTANT);
+  }
+
+  /**
    * Return the result of invoking the '*' operator on this object with the given argument.
    * 
    * @param rightOperand the right-hand operand of the operation