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

- Put pointer to kernel data into Script.
- Replace function.kernel_function pointer to AstNode with
  kernel_offset():
- Replace field.kernel_field pointer to AstNode with kernel_offest().
- Stream the previously unstreamed AstNodes: FunctionDeclaration and
  FunctionExpression.
- Move special handling for _buildin.getMainClosure into the streaming
  flowgraph builder.
- Delete big parts of kernel_to_il.

R=kmillikin@google.com

Committed: https://github.com/dart-lang/sdk/commit/948e2f5d7916032ed6f0bfdc316ba4f577a5ea61
Review-Url: https://codereview.chromium.org/2901533002 .

runtime/vm/kernel_binary.cc

@@ -257,6 +257,7 @@ Typedef* Typedef::ReadFrom(Reader* reader) {
 Class* Class::ReadFrom(Reader* reader) {
   TRACE_READ_OFFSET();
 
+  kernel_offset_ = reader->offset() - 1;  // -1 to include tag byte.
   canonical_name_ = reader->ReadCanonicalNameReference();
   position_ = reader->ReadPosition(false);
   is_abstract_ = reader->ReadBool();
@@ -1843,6 +1844,7 @@ FunctionNode* FunctionNode::ReadFrom(Reader* reader) {
   function->dart_async_marker_ =
       static_cast<FunctionNode::AsyncMarker>(reader->ReadByte());
   function->type_parameters().ReadFrom(reader);
+  reader->ReadUInt();  // total parameter count.
   function->required_parameter_count_ = reader->ReadUInt();
   function->positional_parameters().ReadFromStatic<VariableDeclarationImpl>(
       reader);