Address flan@'s comments on GSS.

The change Iae743842c5707c0231b1c4eb290f7914b6f7a501 has been merged
before flan@ posts all his comments.
This change will address all of his last comments.

Change-Id: I12c7a7afeab4a6a56b19aee2543eaaeacbc62566
Fixes sporadic slow compiles by removing softMap caches.

Several caches were holding only "soft references" to cached objects so
that when under memory pressure the garbage collector could choose to 
clear some of the cached contents. It sounds reasonable but leads to 
hard-to-explain sporadically slow compiles after cache clears.

It seems better to hold onto all references so compiles are always
fast. If the compile runs out of memory it is probably best to throw an
OOM error so that the user will allocate more memory.

Change-Id: I14f180c8c42607c02dd06a1d01d7503adc7e8475
Review-Link: https://gwt-review.googlesource.com/#/c/9840/
(cherry picked from commit 2c0a251b5e9bd4fe027f08661e237987b6215fb9)
Stops enumerating the contents of hidden directories, to save time.

GWT's default resource scanning process already takes some care to 
ignore .svn and similar directories, but does so at a stage after "all"
files have been enumerated in all registered directories. As a result 
the GWT compiler is still paying the full cost of that large file list 
enumeration. This patch changes the file enumeration process to never 
even look inside intentionally hidden folders, thus avoiding any 
resulting file enumeration cost.

Change-Id: Ibd202c17d8287c240660e4f1169643c84489fe1a
Review-Link: https://gwt-review.googlesource.com/#/c/9850/
(cherry picked from commit 398ccf6158885245ae5457047da89c7ac13f0a87)
Adds relative path support to ResourceOracle.

Change-Id: I9b213fbd7411656677e5ff923f4c7f9564fe036d
(cherry picked from commit 6b7ed9b5f244712a5751b20e43b0c34b696a644f)