Add support for the primitives js.import(specifier) and js.import.meta.

These changes are not tested in this repo, because we do not have
the infrastructure for tests requiring ECMAScript Modules. They
should be straightforward enough that it is not really necessary.
All the magic of these two features happens in the linker, not in
the compiler.

Author: sjrd

compiler/src/dotty/tools/backend/sjs/JSCodeGen.scala

@@ -3695,6 +3695,15 @@ class JSCodeGen()(using genCtx: Context) {
         // BoxedUnit.UNIT, which is the boxed version of ()
         js.Undefined()
 
+      case JS_IMPORT =>
+        // js.import(arg)
+        val arg = genArgs1
+        js.JSImportCall(arg)
+
+      case JS_IMPORT_META =>
+        // js.import.meta
+        js.JSImportMeta()
+
       case JS_NATIVE =>
         // js.native
         report.error(

compiler/src/dotty/tools/backend/sjs/JSDefinitions.scala

@@ -145,6 +145,13 @@ final class JSDefinitions()(using Context) {
     @threadUnsafe lazy val JSConstructorTag_materializeR = JSConstructorTagModule.requiredMethodRef("materialize")
     def JSConstructorTag_materialize(using Context) = JSConstructorTag_materializeR.symbol
 
+  @threadUnsafe lazy val JSImportModuleRef = requiredModuleRef("scala.scalajs.js.import")
+  def JSImportModule(using Context) = JSImportModuleRef.symbol
+    @threadUnsafe lazy val JSImport_applyR = JSImportModule.requiredMethodRef(nme.apply)
+    def JSImport_apply(using Context) = JSImport_applyR.symbol
+    @threadUnsafe lazy val JSImport_metaR = JSImportModule.requiredMethodRef("meta")
+    def JSImport_meta(using Context) = JSImport_metaR.symbol
+
   @threadUnsafe lazy val RuntimePackageVal = requiredPackage("scala.scalajs.runtime")
   @threadUnsafe lazy val RuntimePackageClass = RuntimePackageVal.moduleClass.asClass
     @threadUnsafe lazy val RuntimePackage_wrapJavaScriptExceptionR = RuntimePackageClass.requiredMethodRef("wrapJavaScriptException")

compiler/src/dotty/tools/backend/sjs/JSPrimitives.scala

@@ -27,7 +27,10 @@ object JSPrimitives {
 
   final val UNITVAL = JS_NATIVE + 1 // () value, which is undefined
 
-  final val CONSTRUCTOROF = UNITVAL + 1                                // runtime.constructorOf(clazz)
+  final val JS_IMPORT = UNITVAL + 1        // js.import.apply(specifier)
+  final val JS_IMPORT_META = JS_IMPORT + 1 // js.import.meta
+
+  final val CONSTRUCTOROF = JS_IMPORT_META + 1                         // runtime.constructorOf(clazz)
   final val CREATE_INNER_JS_CLASS = CONSTRUCTOROF + 1                  // runtime.createInnerJSClass
   final val CREATE_LOCAL_JS_CLASS = CREATE_INNER_JS_CLASS + 1          // runtime.createLocalJSClass
   final val WITH_CONTEXTUAL_JS_CLASS_VALUE = CREATE_LOCAL_JS_CLASS + 1 // runtime.withContextualJSClassValue
@@ -106,6 +109,9 @@ class JSPrimitives(ictx: Context) extends DottyPrimitives(ictx) {
 
     addPrimitive(defn.BoxedUnit_UNIT, UNITVAL)
 
+    addPrimitive(jsdefn.JSImport_apply, JS_IMPORT)
+    addPrimitive(jsdefn.JSImport_meta, JS_IMPORT_META)
+
     addPrimitive(jsdefn.Runtime_constructorOf, CONSTRUCTOROF)
     addPrimitive(jsdefn.Runtime_createInnerJSClass, CREATE_INNER_JS_CLASS)
     addPrimitive(jsdefn.Runtime_createLocalJSClass, CREATE_LOCAL_JS_CLASS)