[ VM / Service ] Move sourceLocation property to reference objects for Class, Function, and Field.

Token position and script reference information are cheap to provide and
make it possible to tie objects to scripts without requiring additional
requests for full objects.

TEST=Existing

Change-Id: I917714149a72a53081fee5626ccad858e86f5313
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201864
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>

third_party/devtools/update.sh

@@ -36,5 +36,6 @@ cipd create \
   -name dart/third_party/flutter/devtools \
   -in cipd_package \
   -install-mode copy \
+  -preserve-writable \
   -tag git_revision:$1