Makes JsSourceMaps much faster.

Changed the internal storage from Map to List since no callers were
actually needing the Map capabilities.

Started pruning redundant Range instances sooner so that intermediary
JsSourceMaps processors have fewer nodes to examine.

Presized two very-large StringBuilder instances to avoid repeated array
copies during expansion.

Change-Id: Id92dfcdc2e093204b693affc9f39eafc6fb91079
Review-Link: https://gwt-review.googlesource.com/#/c/9051/