Implement new constant expressions

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

Index: editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/StringState.java
diff --git a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/StringState.java b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/StringState.java
index 02c3750410a8b6efa454cf215c74585dc65e02be..2767eda9f9d4fc483c449ebdf7f2eb9d326911af 100644
--- a/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/StringState.java
+++ b/editor/tools/plugins/com.google.dart.engine/src/com/google/dart/engine/internal/object/StringState.java
@@ -15,6 +15,8 @@ package com.google.dart.engine.internal.object;
 
 import com.google.dart.engine.utilities.general.ObjectUtilities;
 
+import java.math.BigInteger;
+
 /**
  * Instances of the class {@code StringState} represent the state of an object representing a
  * string.
@@ -116,6 +118,14 @@ public class StringState extends InstanceState {
   }
 
   @Override
+  public IntState stringLength() throws EvaluationException {
+    if (value == null) {
+      return IntState.UNKNOWN_VALUE;
+    }
+    return new IntState(BigInteger.valueOf(value.length()));
+  }
+
+  @Override
   public String toString() {
     return value == null ? "-unknown-" : "'" + value + "'";
   }