Make processFilesForTarget methods async in caching-compiler.

This ensures that the entire compilation batch runs in its own Fiber,
since each async function invocation recycles a Fiber from the pool.

packages/caching-compiler/caching-compiler.js

@@ -277,7 +277,7 @@ CachingCompiler = class CachingCompiler extends CachingCompilerBase {
   // you have processing you want to perform at the beginning or end of a
   // processing phase, you may want to override this method and call the
   // superclass implementation from within your method.
-  processFilesForTarget(inputFiles) {
+  async processFilesForTarget(inputFiles) {
     const cacheMisses = [];
     const arches = this._cacheDebugEnabled && Object.create(null);
 

packages/caching-compiler/multi-file-caching-compiler.js

@@ -78,7 +78,7 @@ extends CachingCompilerBase {
   }
 
   // The processFilesForTarget method from the Plugin.registerCompiler API.
-  processFilesForTarget(inputFiles) {
+  async processFilesForTarget(inputFiles) {
     const allFiles = new Map;
     const cacheKeyMap = new Map;
     const cacheMisses = [];