Add DISALLOW_COPY_AND_ASSIGN to subclasses of ValueObject and make ValueObject copyable.

runtime/vm/assembler.h

Index: runtime/vm/assembler.h
===================================================================
--- runtime/vm/assembler.h	(revision 18957)
+++ runtime/vm/assembler.h	(working copy)
@@ -39,6 +39,8 @@
  private:
   const char* name_;
   const uword address_;
+
+  DISALLOW_COPY_AND_ASSIGN(ExternalLabel);
 };
 
 
@@ -134,6 +136,7 @@
     intptr_t gap_;
 
     intptr_t ComputeGap() { return buffer_->Capacity() - buffer_->Size(); }
+    DISALLOW_COPY_AND_ASSIGN(EnsureCapacity);
   };
 
   bool has_ensured_capacity_;
@@ -144,6 +147,8 @@
     explicit EnsureCapacity(AssemblerBuffer* buffer) {
       if (buffer->cursor() >= buffer->limit()) buffer->ExtendCapacity();
     }
+   private:
+    DISALLOW_COPY_AND_ASSIGN(EnsureCapacity);
   };
 
   // When building the C++ tests, assertion code is enabled. To allow
@@ -189,6 +194,8 @@
   void ExtendCapacity();
 
   friend class AssemblerFixup;
+
+  DISALLOW_COPY_AND_ASSIGN(AssemblerBuffer);
 };
 
 }  // namespace dart