Unused symbol: do not search for internal member light methods

Use direct references search instead
So #KT-19811 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/UnusedSymbolInspection.kt

@@ -274,7 +274,7 @@ class UnusedSymbolInspection : AbstractKotlinInspection() {
             return false
         }
 
-        if (declaration is KtCallableDeclaration) {
+        if (declaration is KtCallableDeclaration && !declaration.hasModifier(KtTokens.INTERNAL_KEYWORD)) {
             val lightMethods = declaration.toLightMethods()
             if (lightMethods.isNotEmpty()) {
                 return lightMethods.any { method ->

idea/testData/inspectionsLocal/unusedSymbol/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection

idea/testData/inspectionsLocal/unusedSymbol/internal.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+class Foo {
+    fun test() {
+        abacaba("")
+    }
+
+    internal fun <caret>abacaba(s: String): String = s
+}

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -1842,6 +1842,21 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
         }
     }
 
+    @TestMetadata("idea/testData/inspectionsLocal/unusedSymbol")
+    @TestDataPath("$PROJECT_ROOT")
+    @RunWith(JUnit3RunnerWithInners.class)
+    public static class UnusedSymbol extends AbstractLocalInspectionTest {
+        public void testAllFilesPresentInUnusedSymbol() throws Exception {
+            KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/testData/inspectionsLocal/unusedSymbol"), Pattern.compile("^([\\w\\-_]+)\\.kt$"), TargetBackend.ANY, true);
+        }
+
+        @TestMetadata("internal.kt")
+        public void testInternal() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/testData/inspectionsLocal/unusedSymbol/internal.kt");
+            doTest(fileName);
+        }
+    }
+
     @TestMetadata("idea/testData/inspectionsLocal/useExpressionBody")
     @TestDataPath("$PROJECT_ROOT")
     @RunWith(JUnit3RunnerWithInners.class)