[dart2wasm] Allow using table slots for storing values of dart globals

This seems to result in -0.6% compressed main module and a bit less
in uncompressed mode.

Sometimes we have many fields with lazy initializers of the same type.
That led us to emit 1 nullable wasm global for each such field. For
example all proto classes have a `static BuilderInfo i_` field. This has
led to thousands of `(mut (ref null $BuilderInfo))` wasm globals.

Now we use a wasm table for this, which is a O(1) in the binary as they
all get `null` by default, saving us all these globals. The downside is
that accesses have an extra instruction now, but overall this is a win.

The CL also cleans up `globals.dart` by separating the concept of a wasm
global and reading/writing to it from the concept of a Dart global - as
Dart globals can now be backed by wasm globals or table slots.

This CL uses the new capability made possible by the refactoring
in [0] - namely to emit element sections which initialize wasm table
slots with non-function expressions.

[0] https://dart-review.googlesource.com/c/sdk/+/459440

Change-Id: Ie57206dff8c0a57a1df5e48c0808167f822bc4a2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/475800
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>