compiler.wasm.gz contains OpenJDK javac (jdk25u, revision 6c48f4ed), licensed GPLv2 with the Classpath Exception (LICENSE-openjdk.txt). Source: https://github.com/openjdk/jdk25u/tree/6c48f4ed
The compiler and class library are built with teavm-javac and TeaVM 0.16.0 (Apache-2.0, LICENSE-teavm.txt). Source: https://github.com/konsoletyper/teavm-javac and https://github.com/konsoletyper/teavm
Gwecode changes to teavm-javac and TeaVM are in teavm-javac-gwecode.patch.