Fix kotlin -classpath argument

kotlinc expects -classpath, not --classpath.

Also add a test that uses only .kt files to exercise the code when
there are no java sources.

Test: java_test.go
Change-Id: Ifa5a007b460b40ea2188d0907570fbdca6c48da7

java/builder.go

@@ -203,7 +203,7 @@ func TransformKotlinToClasses(ctx android.ModuleContext, outputFile android.Writ
 		Output:      outputFile,
 		Inputs:      inputs,
 		Args: map[string]string{
-			"classpath":    flags.kotlincClasspath.FormJavaClassPath("--classpath"),
+			"classpath":    flags.kotlincClasspath.FormJavaClassPath("-classpath"),
 			"kotlincFlags": flags.kotlincFlags,
 			"outDir":       classDir.String(),
 			"javaVersion":  flags.javaVersion,

java/java_test.go

@@ -631,6 +631,11 @@ func TestKotlin(t *testing.T) {
 			name: "foo",
                         srcs: ["a.java", "b.kt"],
 		}
+
+		java_library {
+			name: "bar",
+                        srcs: ["b.kt"],
+		}
 		`)
 
 	kotlinc := ctx.ModuleForTests("foo", "android_common").Rule("kotlinc")
@@ -655,6 +660,13 @@ func TestKotlin(t *testing.T) {
 		t.Errorf("foo jar inputs %v does not contain %q",
 			jar.Inputs.Strings(), kotlinc.Output.String())
 	}
+
+	kotlinc = ctx.ModuleForTests("bar", "android_common").Rule("kotlinc")
+	jar = ctx.ModuleForTests("bar", "android_common").Output("combined/bar.jar")
+
+	if len(kotlinc.Inputs) != 1 || kotlinc.Inputs[0].String() != "b.kt" {
+		t.Errorf(`bar kotlinc inputs %v != ["b.kt"]`, kotlinc.Inputs)
+	}
 }
 
 func fail(t *testing.T, errs []error) {