[kernel] Add fileOffset on FunctionExpression

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 695 matching lines @@
   Expression value;
 }
 
 type AwaitExpression extends Expression {
   Byte tag = 51;
   Expression operand;
 }
 
 type FunctionExpression extends Expression {
   Byte tag = 52;
+  FileOffset fileOffset;
   FunctionNode function;
 }
 
 type Let extends Expression {
   Byte tag = 53;
   VariableDeclaration variable;
   Expression body;
 }
 
 type LoadLibrary extends Expression {
@@ skipping 310 matching lines @@
   Option<DartType> bound;
 }
 
 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.
 }
 
 ```