Accept constant string concatenation and constant string length as constant

runtime/vm/ast.h

Index: runtime/vm/ast.h
===================================================================
--- runtime/vm/ast.h	(revision 43830)
+++ runtime/vm/ast.h	(working copy)
@@ -129,6 +129,14 @@
     return NULL;
   }
 
+  // Returns true if this node can be a compile-time constant, assuming
+  // that all nodes it depends on are also compile-time constants of
+  // the proper types and values.
+  // See the concept of "potentially constant expression" in the language spec.
+  // The purpose of IsPotentiallyConst is to detect cases where the node is
+  // known NOT to be a constant expression, in which case false is returned and
+  // a compile-time error is reported by the compiler. Otherwise, an error may
+  // still be reported at run-time depending on actual values.
   virtual bool IsPotentiallyConst() const { return false; }
 
   // Analyzes an expression to determine whether it is a compile time
@@ -1423,6 +1431,9 @@
 
   virtual AstNode* MakeAssignmentNode(AstNode* rhs);
 
+  virtual bool IsPotentiallyConst() const;
+  virtual const Instance* EvalConstExpr() const;
+
   DECLARE_COMMON_NODE_FUNCTIONS(InstanceGetterNode);
 
  private: