Do not persist breakpoints across hot restarts or page refreshes

[elliette]

Fixes #2257

• Use setBreakpoint instead of setBreakpointByUrl. When breakpoints are set with the setBreakpoint API, Chrome will not try to persist breakpoints across page reloads. This was causing an issue where Chrome might still be hanging onto a breakpoint that didn't exist in the debugging client.
• Remove the logic to try to re-establish breakpoints after a page reload. This is not what the VM service does. Instead, the debugging client should be in charge of re-establishing the breakpoints.

See go/dart-web-phantom-breakpoints for more details

[nshahan]

LGTM

[elliette]

Submitting with failing frontend_server_client_test. Failure is unrelated to this change, should be fixed with #2376