[dart2js] Easy runtime improvements in SSA.

There are two changes here that account for ~5% of the cycles spent compiling large programs:
1) Calls to 'runtimeType' (specifically _AbstractType.toString) are expensive and CodegenPhase.name calls this 2/3 times per member in the target program.
2) ModularNamer._jsVariableReservedCache is a late final field that gets instantiated for every namer in the program, of which there is one per member in the target program. All the subsets are static and so _jsVariableReservedCache can be static as well. Ideally this would be const but const sets cannot have duplicate strings and the various subsets have some overlaps.

Change-Id: Iefc294145feebe397b2db84a5f49906c6b1372a1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/317981
Reviewed-by: Stephen Adams <sra@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>