Hacky streaming of VariableGet

pkg/kernel/binary.md

 This file describes the binary format of Dart Kernel.
 
 Notation
 --------
 Bitmasks are described with the syntax:
 ```scala
     Byte flags (flag1, flag2, ..., flagN)
 ```
 where 'flag<N>' is the N-th least significant bit,
 (so flag1 is the least significant bit).
@@ skipping 320 matching lines @@
 
 abstract type Expression extends Node {}
 
 type InvalidExpression extends Expression {
   Byte tag = 19;
 }
 
 type VariableGet extends Expression {
   Byte tag = 20;
   FileOffset fileOffset;
+  UInt variableDeclarationPosition; // Notice that this field is temporary.

[Kevin Millikin (Google), 2017/04/05 08:59:57]

I don't think the comment belongs here.  But do say that it is the offset of the
declaration.

   VariableReference variable;
 }
 
 type SpecializedVariableGet extends Expression {
   Byte tag = 128 + N; // Where 0 <= N < 8.
   // Equivalent to a VariableGet with index N.
   FileOffset fileOffset;
+  UInt variableDeclarationPosition; // Notice that this field is temporary.
 }
 
 type VariableSet extends Expression {
   Byte tag = 21;
   FileOffset fileOffset;
   VariableReference variable;
   Expression value;
 }
 
 type SpecializedVariableSet extends Expression {
@@ skipping 582 matching lines @@
   Byte tag = 88;
 }
 
 type TypeParameter {
   // Note: there is no tag on TypeParameter
   StringReference name; // Cosmetic, may be empty, not unique.
   DartType bound; // 'dynamic' if no explicit bound was given.
 }
 
 ```