Light classes: Fix getTextOffset() for identifiers

Before this commit, getTextOffset() and getTextRange() implementations were inconsistent.

compiler/light-classes/src/org/jetbrains/kotlin/asJava/elements/KtLightIdentifier.kt

@@ -46,4 +46,5 @@ open class KtLightIdentifier(
     override fun getParent() = lightOwner
     override fun getContainingFile() = lightOwner.containingFile
     override fun getTextRange() = origin?.textRange ?: TextRange.EMPTY_RANGE
+    override fun getTextOffset(): Int = origin?.textOffset ?: -1
 }

idea/tests/org/jetbrains/kotlin/idea/lightClasses/LightClassBehaviorTest.kt

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
+ */
+
+package org.jetbrains.kotlin.idea.lightClasses
+
+import com.intellij.psi.PsiMethod
+import com.intellij.testFramework.LightProjectDescriptor
+import org.jetbrains.kotlin.idea.test.KotlinLightCodeInsightFixtureTestCase
+import org.jetbrains.kotlin.idea.test.KotlinWithJdkAndRuntimeLightProjectDescriptor
+import org.jetbrains.kotlin.test.JUnit3WithIdeaConfigurationRunner
+import org.junit.runner.RunWith
+
+@RunWith(JUnit3WithIdeaConfigurationRunner::class)
+class LightClassBehaviorTest : KotlinLightCodeInsightFixtureTestCase() {
+    fun testIdentifierOffsets() {
+        myFixture.configureByText(
+            "test.kt", """
+            class A {
+                fun foo() {}
+            }
+            """.trimIndent()
+        )
+
+        val aClass = myFixture.javaFacade.findClass("A")!!
+        val fooMethodName = (aClass.findMethodsByName("foo").single() as PsiMethod).nameIdentifier!!
+
+        val offset = fooMethodName.textOffset
+        val range = fooMethodName.textRange
+
+        assert(offset > 0)
+        assert(offset == range.startOffset)
+    }
+
+    override fun getProjectDescriptor(): LightProjectDescriptor {
+        return KotlinWithJdkAndRuntimeLightProjectDescriptor.INSTANCE
+    }
+}