[kernel] Stream everything. Replace .kernel_function with .kernel_offset

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).
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 }
 */
 
 type FunctionNode {
   Byte tag = 3;
   FileOffset fileOffset;
   FileOffset fileEndOffset;
   Byte asyncMarker; // Index into AsyncMarker above.
   Byte dartAsyncMarker; // Index into AsyncMarker above.
   List<TypeParameter> typeParameters;
+  UInt parameterCount; // positionalParameters.length + namedParameters.length.
   UInt requiredParameterCount;
   List<VariableDeclaration> positionalParameters;
   List<VariableDeclaration> namedParameters;
   DartType returnType;
   Option<Statement> body;
 }
 
 type VariableReference {
   // Reference to the Nth variable in scope, with 0 being the
   // first variable declared in the outermost scope, and larger
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   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.
 }
 
 ```