impl: avoid loading noopclasses (#201)

Tested with the Benchmark allowed to load the java7/9 packages

```
BEFORE

Benchmark                                             Mode  Cnt     Score     Error        Units
ClassInitBenchmark.forName_init                         ss  400  8730.958 ± 142.833        us/op
ClassInitBenchmark.forName_init:·class.load             ss  400     1.166 ±   0.015  classes/sec
ClassInitBenchmark.forName_init:·class.load.norm        ss  400    72.105 ±   0.112   classes/op
ClassInitBenchmark.forName_init:·class.unload           ss  400       ≈ 0            classes/sec
ClassInitBenchmark.forName_init:·class.unload.norm      ss  400       ≈ 0             classes/op
ClassInitBenchmark.forName_noinit                       ss  400  3040.308 ±  42.065        us/op
ClassInitBenchmark.forName_noinit:·class.load           ss  400     0.605 ±   0.006  classes/sec
ClassInitBenchmark.forName_noinit:·class.load.norm      ss  400    57.845 ±   0.096   classes/op
ClassInitBenchmark.forName_noinit:·class.unload         ss  400       ≈ 0            classes/sec
ClassInitBenchmark.forName_noinit:·class.unload.norm    ss  400       ≈ 0             classes/op

AFTER

Benchmark                                             Mode  Cnt     Score     Error        Units
ClassInitBenchmark.forName_init                         ss  400  8411.036 ± 148.305        us/op
ClassInitBenchmark.forName_init:·class.load             ss  400     1.136 ±   0.016  classes/sec
ClassInitBenchmark.forName_init:·class.load.norm        ss  400    71.040 ±   0.113   classes/op
ClassInitBenchmark.forName_init:·class.unload           ss  400       ≈ 0            classes/sec
ClassInitBenchmark.forName_init:·class.unload.norm      ss  400       ≈ 0             classes/op
ClassInitBenchmark.forName_noinit                       ss  400  3077.689 ±  52.719        us/op
ClassInitBenchmark.forName_noinit:·class.load           ss  400     0.612 ±   0.007  classes/sec
ClassInitBenchmark.forName_noinit:·class.load.norm      ss  400    57.815 ±   0.093   classes/op
ClassInitBenchmark.forName_noinit:·class.unload         ss  400       ≈ 0            classes/sec
ClassInitBenchmark.forName_noinit:·class.unload.norm    ss  400       ≈ 0             classes/op
```

Author: carl-mastrangelo

impl/src/main/java/io/perfmark/impl/SecretPerfMarkImpl.java

@@ -77,10 +77,15 @@ public static final class PerfMarkImpl extends Impl {
           problems[2] = t;
         }
       }
-      if (gen == null) {
-        generator = new NoopGenerator();
-      } else {
+      if (gen != null) {
         generator = gen;
+      } else {
+        // This magic incantation avoids loading the NoopGenerator class.   When PerfMarkImpl is
+        // being verified, the JVM needs to load NoopGenerator to see that it actually is a
+        // Generator.  By doing a cast here, Java pushes the verification to when this branch is
+        // actually taken, which is uncommon.  Avoid reflectively loading the class, which may
+        // make binary shrinkers drop the NoopGenerator class.
+        generator = (Generator) (Object) new NoopGenerator();
       }
 
       boolean startEnabled = false;

impl/src/main/java/io/perfmark/impl/Storage.java

@@ -90,10 +90,15 @@ public final class Storage {
         problems[2] = t;
       }
     }
-    if (provider == null) {
-      markRecorderProvider = new NoopMarkRecorderProvider();
-    } else {
+    if (provider != null) {
       markRecorderProvider = provider;
+    } else {
+      // This magic incantation avoids loading the NoopMarkRecorderProvider class.   When Storage
+      // is being verified, the JVM needs to load NoopMarkRecorderProvider to see that it actually
+      // is a MarkRecorderProvider.  By doing a cast here, Java pushes the verification to when
+      // this branch is actually taken, which is uncommon.  Avoid reflectively loading the class,
+      // which may make binary shrinkers drop the NoopMarkRecorderProvider class.
+      markRecorderProvider = (MarkRecorderProvider) (Object) new NoopMarkRecorderProvider();
     }
     try {
       if (Boolean.getBoolean("io.perfmark.PerfMark.debug")) {