Make clutz support Closure's BROWSER_WITH_TRANSFORMED_PREFIXES resolution.

Previously, clutz would throw when encountering an es6 file with
absolute import. Closure has support for such paths along with an option
to rewrite them to something rooted using the
BROWSER_WITH_TRANSFORMED_PREFIXES module resolution strategy.

This change makes clutz use that strategy and makes it so that one can
configure which absolute prefixes will be replaced with '/'.

Note, this is still not proper support for ES6, but rather just making
sure clutz doesn't reject code that Closure could accept using the right
flags.

Author: rkirov

src/main/java/com/google/javascript/clutz/Options.java

@@ -4,10 +4,12 @@
 import static java.nio.charset.StandardCharsets.UTF_8;
 
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Sets;
 import com.google.common.io.Files;
 import com.google.javascript.jscomp.*;
 import com.google.javascript.jscomp.CompilerOptions.LanguageMode;
+import com.google.javascript.jscomp.deps.ModuleLoader;
 import com.google.javascript.jscomp.parsing.Config;
 import java.io.File;
 import java.io.IOException;
@@ -163,6 +165,13 @@ public int parseArguments(Parameters params) throws CmdLineException {
   )
   private CompilerOptions.TracerMode tracerMode = CompilerOptions.TracerMode.OFF;
 
+  @Option(
+    name = "--browserResolverStrippedPrefixes",
+    usage = "A list of prefixes for absolute ES6 module paths, that would be replaced by '/'",
+    handler = StringArrayOptionHandler.class
+  )
+  List<String> browserResolverStrippedPrefixes = new ArrayList<>();
+
   @Argument
   @Option(name = "--", handler = StopOptionHandler.class)
   List<String> arguments = new ArrayList<>();
@@ -208,6 +217,14 @@ public CompilerOptions getCompilerOptions() {
     // Always parse and analyze the latest language features closure supports.
     options.setLanguage(LanguageMode.ECMASCRIPT_NEXT);
     options.setLanguageOut(LanguageMode.ECMASCRIPT5);
+
+    options.setModuleResolutionMode(ModuleLoader.ResolutionMode.BROWSER_WITH_TRANSFORMED_PREFIXES);
+    ImmutableMap.Builder builder = ImmutableMap.builder();
+    for (String str : browserResolverStrippedPrefixes) {
+      builder.put(str, "/");
+    }
+    options.setBrowserResolverPrefixReplacements(builder.build());
+
     if (closureEnv != null) {
       options.setEnvironment(closureEnv);
     }

src/test/java/com/google/javascript/clutz/ProgramSubject.java

@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.io.PrintStream;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -124,6 +125,7 @@ private String[] parse() throws AssertionError {
       opts.partialInput = true;
     }
     opts.collidingProvides = ImmutableSet.of("colliding_provide.aliased");
+    opts.browserResolverStrippedPrefixes = Arrays.asList("abs_strip_for_testing");
 
     List<SourceFile> sourceFiles = new ArrayList<>();
 

src/test/java/com/google/javascript/clutz/partial/es6_module.d.ts

@@ -0,0 +1,10 @@
+import * as rel from './rel/module/name';
+import * as rel2 from '/abs/module/name';
+//!! abs_strip_for_testing is special cased in the test harness.
+import * as abs from 'abs_strip_for_testing/module/name';
+
+export let x = rel.x + abs.x + rel2.x;
+
+//!! The emit of this file is intentionally empty.
+//!! So far clutz doesn't support es6 modules properly, but the test
+//!! makes sure that it doesn't reject them either.