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
diff --git a/java/java_test.go b/java/java_test.go
index 8ac4b3b..b819447 100644
--- a/java/java_test.go
+++ b/java/java_test.go
@@ -631,6 +631,11 @@
 			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 @@
 		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) {