Replace dx with d8

d8 desugaring of Java 8 / Java 11 features seems to work without any --library arguments
despite d8's warnings. The resulting smali code no longer contains 'invoke-custom' statements.
Example: String#isBlank (Java 11) is replaced by a $$ExternalSyntheticBackport class.

src/main/kotlin/org/ollide/java2smali/D8Class2DexHelper.kt

@@ -0,0 +1,45 @@
+package org.ollide.java2smali
+
+import com.android.tools.r8.D8
+import com.android.tools.r8.D8Command
+import com.android.tools.r8.OutputMode
+import com.intellij.openapi.diagnostic.Logger
+import com.intellij.util.containers.stream
+import java.nio.file.Path
+import java.nio.file.Paths
+import java.util.stream.Collectors
+
+object D8Class2DexHelper {
+
+    /**
+     * Uses the D8 tool from the Android Build Tools to create
+     * a .dex version of a compiled java file (.class)
+     *
+     * @param inputClassFilePaths full paths to the compiled .class file
+     * @param outputDexPath this will be the dex output file's path and name
+     * @return `true` if dx ran successfully, otherwise `false`
+     */
+    fun dexClassFile(inputClassFilePaths: Array<String>, outputDexPath: Path): Boolean {
+        val classFiles = inputClassFilePaths.stream().map { Paths.get(it) }.collect(Collectors.toList())
+
+        val command = D8Command.builder()
+                .setIntermediate(true)
+                .setMinApiLevel(30)
+                .addProgramFiles(classFiles)
+                // Add --library android.jar ?
+                // .addLibraryFiles(Paths.get(""))
+                .setOutput(outputDexPath, OutputMode.DexIndexed)
+                .build()
+
+        return try {
+            D8.run(command)
+            true
+        } catch (e: Exception) {
+            LOG.error("Error running D8", e)
+            false
+        }
+    }
+
+    private val LOG = Logger.getInstance(D8Class2DexHelper::class.java)
+
+}

src/main/kotlin/org/ollide/java2smali/DexCompiler.kt

@@ -54,19 +54,21 @@ class DexCompiler(private val vFile: VirtualFile, private val project: Project,
         fileOutputDirectory.refresh(false, false)
 
         val fileName = vFile.nameWithoutExtension
-        val dexFilePath = Paths.get(fileOutputDirectory.path, fileName + DEX_EXTENSION).toString()
+        val dexFilePath = Paths.get(fileOutputDirectory.path)
 
         // CLASS -> DEX
         val targetFiles = getClassFiles(fileOutputDirectory, fileName)
-        val successfulDex = Class2DexHelper.dexClassFile(targetFiles, dexFilePath)
+        val successfulDex = D8Class2DexHelper.dexClassFile(targetFiles, dexFilePath)
         if (!successfulDex) {
             return
         }
 
+        val dexFile = dexFilePath.resolve("classes$DEX_EXTENSION").toString()
+
         // DEX -> SMALI
         val outputDir = getSourceRootFile().path
         WriteCommandAction.runWriteCommandAction(project) {
-            Dex2SmaliHelper.disassembleDexFile(dexFilePath, outputDir)
+            Dex2SmaliHelper.disassembleDexFile(dexFile, outputDir)
 
             // we've created the smali file(s) in our source file's directory
             // refresh directory synchronously and access children to let IDEA detect the file(s)