[dart2js] Don't store empty variable map in KernelToLocalsMap
[dart2js] Don't store empty variable map in KernelToLocalsMap

Reduces dart2js heap for link phase of modular compile by ~100MB.

Change-Id: I2037c2010efe055b2564394f091206069a54de5e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177220
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Commit-Queue: Stephen Adams <60cb5b73b7e0cb7dcda53302da4fab6ecfc7fa8b@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk