fix(ComponentUrlMapper): support relative template URLs in Dartium

When running in Dartium without using transformers (i.e. with a normal
static web server), handle relative template URLs. This works by using
mirrors to get the URL of the library where the component class is
defined.

Closes #2771

Closes #3743

modules_dart/transform/lib/src/transform/template_compiler/reflection/reflection_capabilities.dart

@@ -10,9 +10,7 @@ class NullReflectionCapabilities implements ReflectionCapabilities {
 
   _notImplemented(String name) => throw 'Not implemented: $name';
 
-  bool isReflectionEnabled() {
-    return false;
-  }
+  bool isReflectionEnabled() => false;
 
   Function factory(Type type) => _notImplemented("factory");
 
@@ -27,6 +25,8 @@ class NullReflectionCapabilities implements ReflectionCapabilities {
   SetterFn setter(String name) => _nullSetter;
 
   MethodFn method(String name) => _nullMethod;
+
+  String importUri(Type type) => './';
 }
 
 _nullGetter(Object p) => null;