[FIR] Add extracting contracts in raw fir builder and light tree builder

compiler/fir/tree/src/org/jetbrains/kotlin/fir/FirRenderer.kt

@@ -544,7 +544,11 @@ class FirRenderer(builder: StringBuilder, private val mode: RenderMode = RenderM
     }
 
     override fun visitStatement(statement: FirStatement) {
-        visitElement(statement)
+        if (statement is FirStubStatement) {
+            print("[StubStatement]")
+        } else {
+            visitElement(statement)
+        }
     }
 
     override fun visitReturnExpression(returnExpression: FirReturnExpression) {

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirExpressionUtil.kt

@@ -84,4 +84,13 @@ fun <D> FirBlock.transformAllStatementsExceptLast(transformer: FirTransformer<D>
             TransformData.Nothing
         }
     }
+}
+
+fun FirBlock.replaceFirstStatement(statement: FirStatement): FirStatement {
+    require(this is FirBlockImpl) {
+        "replaceFirstStatement should not be called for ${this::class.simpleName}"
+    }
+    val existed = statements[0]
+    statements[0] = statement
+    return existed
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirStubStatement.kt

@@ -0,0 +1,32 @@
+/*
+ * Copyright 2010-2020 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.fir.expressions.impl
+
+import org.jetbrains.kotlin.fir.FirElement
+import org.jetbrains.kotlin.fir.FirPureAbstractElement
+import org.jetbrains.kotlin.fir.FirSourceElement
+import org.jetbrains.kotlin.fir.expressions.FirAnnotationCall
+import org.jetbrains.kotlin.fir.expressions.FirStatement
+import org.jetbrains.kotlin.fir.visitors.FirTransformer
+import org.jetbrains.kotlin.fir.visitors.FirVisitor
+
+object FirStubStatement : FirPureAbstractElement(), FirStatement {
+    override val source: FirSourceElement?
+        get() = null
+
+    override val annotations: List<FirAnnotationCall>
+        get() = emptyList()
+
+    override fun <D> transformAnnotations(transformer: FirTransformer<D>, data: D): FirStatement {
+        return this
+    }
+
+    override fun <R, D> acceptChildren(visitor: FirVisitor<R, D>, data: D) {}
+
+    override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement {
+        return this
+    }
+}