Auto-assign proto ids using field index

Fixes #70
diff --git a/runtime/common/src/main/kotlin/kotlinx/serialization/protobuf/ProtoBuf.kt b/runtime/common/src/main/kotlin/kotlinx/serialization/protobuf/ProtoBuf.kt
index 9a012ff..40c611c 100644
--- a/runtime/common/src/main/kotlin/kotlinx/serialization/protobuf/ProtoBuf.kt
+++ b/runtime/common/src/main/kotlin/kotlinx/serialization/protobuf/ProtoBuf.kt
@@ -1,5 +1,5 @@
 /*
- * Copyright 2017 JetBrains s.r.o.
+ * Copyright 2018 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -387,7 +387,7 @@
         }
 
         private fun KSerialClassDesc.getProtoDesc(index: Int): ProtoDesc {
-            val tag = this.getAnnotationsForIndex(index).filterIsInstance<SerialId>().single().id
+            val tag = this.getAnnotationsForIndex(index).filterIsInstance<SerialId>().onlySingleOrNull()?.id ?: index
             val format = this.getAnnotationsForIndex(index).filterIsInstance<ProtoType>().onlySingleOrNull()?.type
                     ?: ProtoNumberType.DEFAULT
             return tag to format
diff --git a/runtime/common/src/test/kotlin/kotlinx/serialization/protobuf/AutoAssignIdsTest.kt b/runtime/common/src/test/kotlin/kotlinx/serialization/protobuf/AutoAssignIdsTest.kt
new file mode 100644
index 0000000..e84100f
--- /dev/null
+++ b/runtime/common/src/test/kotlin/kotlinx/serialization/protobuf/AutoAssignIdsTest.kt
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2018 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package kotlinx.serialization.protobuf
+
+import kotlinx.serialization.SerialId
+import kotlinx.serialization.Serializable
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class AutoAssignIdsTest {
+    @Serializable
+    data class WithoutIds(val a: Int, val b: String)
+
+    @Serializable
+    data class WithId(@SerialId(0) val a: Int, @SerialId(1) val b: String)
+
+    @Test
+    fun saveAndRestoreWithoutIds() {
+        val w1 = WithoutIds(1, "foo")
+        val bytes = ProtoBuf.dump(w1)
+        val w2 = ProtoBuf.load<WithoutIds>(bytes)
+        assertEquals(w1, w2)
+    }
+
+    @Test
+    fun incrementalIds() {
+        val w1 = WithoutIds(1, "foo")
+        val bytes = ProtoBuf.dump(w1)
+        val w2 = ProtoBuf.load<WithId>(bytes)
+        assertEquals(w1.a, w2.a)
+        assertEquals(w1.b, w2.b)
+    }
+}