Attempt to synthesize companion class only when needed

Bug: 183998768
Change-Id: I5006b96e4aacd6cc3b10a74b7f92fd5365ba8764

src/main/java/com/android/tools/r8/ir/desugar/itf/EmulatedInterfaceProcessor.java

diff --git a/src/main/java/com/android/tools/r8/ir/desugar/itf/EmulatedInterfaceProcessor.java b/src/main/java/com/android/tools/r8/ir/desugar/itf/EmulatedInterfaceProcessor.java
index e8f4756..9efdc49 100644
--- a/src/main/java/com/android/tools/r8/ir/desugar/itf/EmulatedInterfaceProcessor.java
+++ b/src/main/java/com/android/tools/r8/ir/desugar/itf/EmulatedInterfaceProcessor.java
@@ -23,6 +23,7 @@
 import com.android.tools.r8.utils.Pair;
 import com.android.tools.r8.utils.StringDiagnostic;
 import com.android.tools.r8.utils.collections.ProgramMethodSet;
+import com.google.common.collect.Iterables;
 import com.google.common.collect.Sets;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -138,9 +139,8 @@
     assert rewriter.isEmulatedInterface(emulatedInterface.type);
     DexProgramClass theProgramInterface = emulatedInterface.asProgramClass();
     DexProgramClass synthesizedClass = synthesizeEmulateInterfaceLibraryClass(theProgramInterface);
-    if (synthesizedClass != null) {
-      syntheticClasses.put(emulatedInterface, synthesizedClass);
-    }
+    assert synthesizedClass != null;
+    syntheticClasses.put(emulatedInterface, synthesizedClass);
   }
 
   private DexProgramClass synthesizeEmulateInterfaceLibraryClass(DexProgramClass theInterface) {
@@ -220,9 +220,7 @@
                   extraDispatchCases,
                   appView));
         });
-    if (emulationMethods.isEmpty()) {
-      return null;
-    }
+    assert !emulationMethods.isEmpty();
     DexType emulateLibraryClassType =
         InterfaceMethodRewriter.getEmulateLibraryInterfaceClassType(
             theInterface.type, appView.dexItemFactory());
@@ -280,7 +278,13 @@
         || appView.isAlreadyLibraryDesugared(emulatedInterface)) {
       return;
     }
-    generateEmulateInterfaceLibrary(emulatedInterface);
+    if (needsEmulateInterfaceLibrary(emulatedInterface)) {
+      generateEmulateInterfaceLibrary(emulatedInterface);
+    }
+  }
+
+  private boolean needsEmulateInterfaceLibrary(DexProgramClass emulatedInterface) {
+    return Iterables.any(emulatedInterface.methods(), DexEncodedMethod::isDefaultMethod);
   }
 
   @Override