JsChecker: support extensionless ES6 imports

Author: Rob Figueiredo

closure/compiler/test/BUILD

@@ -237,3 +237,27 @@ sh_test(
         ":exports_data",
     ],
 )
+
+# TEST: ES6 module can import another via a workspace-relative extensionless import path.
+
+closure_js_library(
+    name = "es6module1",
+    srcs = ["es6module1.js"],
+    no_closure_library = True,
+)
+
+closure_js_library(
+    name = "es6module2",
+    srcs = ["es6module2.js"],
+    #    suppress = ["moduleLoad"],
+    no_closure_library = True,
+    deps = [":es6module1"],
+)
+
+closure_js_binary(
+    name = "es6module_bin",
+    compilation_level = "ADVANCED",
+    entry_points = ["/closure/compiler/test/es6module2"],
+    language = "ECMASCRIPT5",
+    deps = [":es6module2"],
+)

java/com/google/javascript/jscomp/CheckStrictDeps.java

@@ -136,6 +136,12 @@ private void visitEs6Import(NodeTraversal t, Node n) {
     private void checkNamespaceIsProvided(NodeTraversal t, Node n, String namespace) {
       if (namespace.startsWith("/") || namespace.startsWith(".")) {
         // TODO(jart): Unify path resolution with ModuleLoader.
+        // NOTE(robfig): To enable usage of extensionless ES6 modules,
+        // copy these 3 lines from NodeModuleResolver.java.
+        if (!namespace.endsWith(".js")) {
+          namespace += ".js";
+        }
+
         Webpath me = Webpath.get(t.getSourceName());
         if (!me.isAbsolute()) {
           me = Webpath.get("/").resolve(me);